In addition to angle-measuring instruments, which allow an angle measurement at a rotatable shaft in incrementable measuring steps, so-called absolute angle-measuring instruments—also referred to as rotary encoders—are conventional as well. These allow an absolute angle determination within a single shaft rotation. Furthermore, if the number of performed shaft rotations is to be recorded as well, so-called multiturn rotary encoders are generally utilized. With such multiturn rotary encoders, the absolute angular position is determined within one shaft rotation, i.e., between 0° and 360°, via a code disk connected to the shaft, which is scanned with the aid of a suitable photoelectric scanning unit. To obtain the required information regarding the number of effected shaft rotations, a reduction or step-down gear is typically provided, via which one or more additional graduated disks or code disks is/are set into a rotary motion having a low number of revolutions per unit of time, while the shaft is rotating. These code disks are often designed as magnetized disks, each having at least one north-pole and one south-pole sector. The rotational position of these additional code disks is normally recorded in a conventional manner with the aid of suitable scanning units, in particular Hall sensors. Due to the specified reduction of the rotary motion of the additional code disks, it is possible to ascertain the number of effected rotations of the shaft in this manner. A measurement of the absolute position of the driven shaft may thus be carried out also over a plurality of rotations.
It may be advantageous in such reduction gearings if the first reduction step utilizes the highest possible reduction ratio so that the gear wheels of the subsequent gearing steps rotate at an already considerably lower rotational frequency. This significantly reduces the loading of the subsequent gear steps.
A similarly configured multiturn rotary encoder is described in German Published Patent Application No. 198 20 014, of the applicant hereof. This describes an integrated construction method for electronic components of rotary encoders.
German Published Patent Application No. 197 45 177 describes a gearing system in which permanently magnetic or ferromagnetic, spiral-shaped segments are arranged along the peripheral areas of a drive gear and an output gear. This design may be disadvantageous insofar as it requires relatively large installation space, in particular as far as the outer dimensions in the radial direction are concerned.
The demand for rotary encoders having smaller structural dimensions continues to grow. Since the electronic components of rotary encoders are integrated to an ever greater extent and are thus miniaturized as well, the installation space for the mechanical components of these devices often becomes a limiting factor in such miniaturization endeavors.